This invention relates to materials testing, and more particularly, to strain responsive devices which detect the expansion and elongation of structural members resulting, for example, from crack growth in such members.
Most structural materials weaken with the stresses of continuous use and eventually fail. In the aircraft industry, finding stressed and weakened structural parts before failure is of great importance, because of the catastrophic impact of an operational failure. In the past, aircraft have been periodically removed from service for structural testing. Much of the structural testing is laborious due to the "hidden" nature of the components undergoing tests; out-of-service time adds greatly to the operational expense of the aircraft. Materials testing of operational aircraft is presently being proposed to improve the timeliness of repair and to reduce the lost time devoted to periodic complete testing.
One type of operational testing is the measurement of the separation of normally connected components, such as sections of aircraft skin, and the measurement and detection of cracks in the skin and structure of the airframe. One separation or crack detection arrangement comprises a plurality of resistor strands which are connected in parallel between the terminals of a resistance measuring circuit. The resistive strands are attached to the structure near an existing joint or expected crack area. As the crack or separation widens, the resistor strands are broken, changing the resistance presented to the measuring circuit. The change of resistance, as detected by the measuring circuit, thus indicates the advancement of the crack.
Conventional resistance measurement devices are subject to corrosion and to other problems due to their electronic nature. For example, electrical resistance crack testing can give extraneous readings or the system can fail because of lightning and substantial electromagnetic pulses. Also, the placement of electrical devices on the skin of an aircraft can create significant electromagnetic interference, which is particularly undesirable for military aircraft.
The problems with resistive crack detection have been avoided with other technologies such as fiber optics. U.S. Pat. No. 4,636,638 to Huang, et al., uses a single fiber optic strand which is secured to a surface under test at numerous points. A crack causes separation of the points, stretching the fiber. When the fiber breaks, optical transmission through the fiber is terminated. Thus, when no signal is detected at an output of the fiber, a crack is suspected. This system does not allow monitoring of crack propagation (i.e., growth).
Systems have been proposed, such as disclosed in U.S. Pat. No. 4,836,030 to Martin, which use a plurality of optical fibers each connected to an optical source at one end and to a separate detector at the other end. As a crack advances, the fibers break one at a time and the individual detectors sense when their associated fiber is broken. Output signals of the separate detectors are used to indicate the advancement of a crack by the progression of the detector output signals. Arrangements of the type disclosed in Martin overcome the corrosion and electrical signal problems of resistive detection systems, but the arrangements are large due to the use of multiple optical detectors and multiple optical fibers between a source and the detectors. The large size of such multiple independent fiber systems is incompatible with the space and weight requirements of modern aircraft.
A need exists for a simple, lightweight crack sensing arrangement which avoids the problems of electronic/resistive detection systems while permitting accurate crack localization and progression measurement.